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View the document Ecological Changes In Lake Victoria After The Invasion Of Nile Perch (La Tes Niloticus):The Catchment, Water Quality,And Fisheries Management

Ecological Changes In Lake Victoria After The Invasion Of Nile Perch (La Tes Niloticus):The Catchment, Water Quality,And Fisheries Management

P.B.O. Ochumba, M. Goshen, And U. Pollingher

Abstract

Lake Victoria, the second largest lake in the world, has undergone dramatic changes since the 1920s. Some of these include: intensive non-selective fisheries; severe alteration of the drainage basin through agricultural, vegetational, and industrialization changes; introduction and invasion of exotic fish species that has led to the elimination of native species; and a progressive buildup of physico-chemical changes in the lake environment. Current studies of Lake Victoria have identified substantial increases in chlorophyll concentration and primary productivity, as well as decreases in silica compared to values measured 30 years ago. Present sulphate concentrations (0.1 mg/l) are lower than the lowest values reported from other large lakes in the world. There has been a shift in the phytoplankton community toward a dominance of blue-greens. The zooplankton densities are relatively low and the body sizes of the organisms are small. Anoxic waters have recently been found at shallower depths than previously reported in Lake Victoria, suggesting significant increases of oxygen demand in the seasonally formed hypolimnion. Algal blooms have also become more extensive in Lake Victoria. Fisheries management in Lake Victoria has led to a shift from relying on a multi-species stock (400-500 haplochromids) to one relying on only two major exotic species, Lates niloticus and Oreochromis niloticus and the endemic Rastrineobola argentea. The present practices of lake management have not improved water quality.

Introduction

Lake Victoria (100 m maximum depth), located in the Syrian-African Rift Valley was created after tectonic activity that formed westward-flowing rivers in the lake basin (Talling 1966, Serruya and Pollingher 1983, Bugenyi and Balirwa 1989). The catchment area (193,000/km²) (Figure 1) and the wet drainage basin is covered by grassland savanna, agricultural crops, and the forested mountains of Rwanda and Burundi. The Kagera and Nzoia are the main inflowing rivers and the outflow is the River Nile. The lake is the second largest in the world and a major source of protein for 10 million people in Kenya (Ochumba 1984). The lake is seasonally stratified (Fish 1957) and wind is the major factor that determines the annual thermal cycle and water column mixing (Newell 1960, Talling 1969).


FIGURE 1. Kenyan portion of Lake Victoria showing drainage area sampling stations and land sources of nutritions

The long water residence time (23.4 years) increases vulnerability to long-term changes caused by environmental modification in the catchment area.

Lake Victoria has undergone successive disruptions since the early 1920s. Major changes in the ecosystem are: intensive nonselective fisheries, modification of the vegetation in the drainage basin, Nile perch (Lates niloticus) invasion and introduction of other exotic fish species, and the progression of physico-chemical changes in the lake. One of the dramatic changes is the development of a seasonal and lake-wide anaerobic hypolimnion which now threatens the integrity and biodiversity of this ecosystem. The endemic fish community of haplochromids has undergone a reduction in abundance and species diversity. The exotic Nile perch currently dominates the commercial catch together with the exotic Oreochromis niloticus and the small endemic cyprinid Rastrineobola argentea. Periodic fish kills in the lake raised serious concern about the environment of Lake Victoria and the impact of developmental activities in the lake basin (Ochumba 1987). Pollution from industrial, agricultural, and urban sources has increased significantly and the physical alteration of the lake shores through construction is proposed.

Changes In The Fish Community And Fisheries

The nearshore fisheries have been dramatically altered since 1920 in term of species composition and reduction of fish stocks. At the beginning of the century, fishing efforts were determined by subsistence requirements using traditional gear (Acere 1988), which was subsequently replaced by the introduction of cotton, nylon, and multifilament gill nets. The Lake Victoria fisheries, as presented by Graham (1929) and Fryer and Iles (1972), described Oreochromis esculenta and O. variabilis, over 250 species of haplochromine species, mormyrids, catfish, and cyprinids as major contributors to the commercial landings. The introduction of gill nets resulted in a drop in catch per unit effort and initiated overfishing conditions (Acere 1988). It was accompanied by the introduction of four species of exotic tilapias, Ti1apia zilli, O. niloticus, O. Ieucostictus, and O. mossambicus. The objectives of these introductions were to increase fish production. Nevertheless, since then the fisheries, in terms of catch per effort, has fluctuated (Garrod 1961).

TABLE 1 Fish Landings in the Kenya Waters of Lake Victoria from 1969 to 1989 (Percents of Total Tons)

Genus

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

Lates

0.1

0.3

0.9

0.13

1.0

14.0

59.4

67.7

56.5

69.1

54.3

Haplochromis

36.8

32.0

33.2

27.9

32.4

21.6

2.4

0.8

0.0

0.03

1.5

Rastrineobola

2.5

5.1

10.5

27.4

34.7

30.5

20.0

21.3

29.2

24.5

38.5

Tilapia

26.6

21.1

10.1

3.9

7.4

9.0

10.2

5.5

10.7

2.8

2.3

Clarias

7.6

12.5

15.7

15.6

9.1

10.0

2.6

2.7

0.6

0.6

1.4

Bagrus

5.5

7.1

8.6

8.4

6.0

5.8

1.1

3.1

0.1

0.0

0.1

Protopterus

9.3

12.8

13.0

1.1

4.0

1.5

0.5

0.3

0.3

0.1

0.1

Schilbe

1.4

0.4

0.9

0.3

0.7

1.0

0.1

0.0

0.0

0.0

0.3

Alestes

0.3

0.4

0.01

-

-

-

0.0

0.0

0.0

0.0

0.0

Barbus

1.1

1.6

1.1

1.7

1.0

1.4

0.8

0.1

0.1

0.2

0.0

Labeo

2.7

1.5

0.8

0.7

0.6

1.8

0.6

0.1

-

0

0.1

Mormyrus

0.4

0.5

1.1

0.3

0.5

1.2

0.5

0.3

0.0

0

0.1

Synodontis

1.5

0.7

1.1

0.8

1.6

1.6

1.3

0.1

-

0

0.2

Small mixed

3.6

5.2

3.8

-

-

-

-

-

2.6

2.4

0.9

 

Prior to the introduction of the Nile perch (early 1960s), haplochromine fishes were the economic basis for the local fishery, as well as the dominant protein resource for human consumption. Detritivore and planktivore fishes were major prey species for the piscivore Nile perch (Fryer 1973, Ogari and Dadzie 1988). In 1976, only 0.5% of the commercial catch in the Kenya waters of Lake Victoria was Nile perch; by 1983 it was up to 67.7%. Haplochromines in the catches varied between 21.6 and 36.8% during 1968-1979 and 0.8% in 1985 (Table 1) (Ogutu-Ohwayo 1985). The landings of other native nearshore species like Tilapia, Protopterus, and Clarias declined as well (Okemwa 1981, FAO 1987). The present drastic reductions in nonpredatory commercial fishes (Table 1) predictively will be accompanied by a decline in Nile perch (Ssentongo and Welcomme 1985) due to its cannibalistic behavior (Barer et al. 1985, Ogari 1985).

Trawl surveys during the 1970s in Lake Victoria (Kudhongania and Cordone 1974, Benda 1981, Muller and Benda 1981) indicated large stocks of haplochromines and several endemic fishes in the nearshore and offshore (Table 2). Trawl fishing during the 1980s (Asila and Ogari 1988, Okemwa 1984) indicated that endemic cyprinids, catfishes, lungfishes, and mormyrids were too scarce to support a significant fishery (Table 2). Consequently, it was concluded that both haplochromines and tilapiines in Lake Victoria were commercially extinct (Balon and Bruton 1986, Ribbink 1987, Les Kaufman, personal communication). The decline of haplochromine and tilapiine fishes resulted not only in a reduction of commercial catch, but also in a disruption of trophic dynamics in the Lake ecosystem. These fishes were consumers of the dominant and bloom-forming algae and detritus and their high feeding capacity prevented water quality deterioration. Their absence was partly responsible for recent algal blooms (Ochumba and Kibaara 1989) and detritus accumulation in the deep layers, followed by an increase in anoxia.

TABLE 2 Trawl Catches of Dominant Species from 1969 to 1990 (Kg per Haul)

Species

1969-1970

1975

1977

1982-1983

1989-1990

 

(19 hauls)

(69 hauls)

(167 hauls)

(54 hauls)

(41 hauls)

Bagrus docmac

11.7

12.5

1.80

0.90

0.01

Clarias gariepinus

3.3

2.6

0.70

0.90

0.10

Haplochromis spp.

35.8

32.7

28.7

0.01

0.54

Labeo victorianus

0.10

0.1

0.1

0.10

0.10

Lates niloticus

0

0.8

2.8

29.0

32.7

Protopterus aethiopicus

3.7

10.7

0.30

0.01

0.01

Schilbe mystus

0.03

0.20

0.01

0.01

0.1

Synodontis spp.

2.10

0.20

0.50

0.04

0.10

Oreochromis variabilis

0.03

0.11

0.30

-

-

Oreochromis niloticus

0.01

0.20

0.70

1.40

1.70

Rastrineobota argentea

x

x

x

x

3.7

x -- not recorded, or not available in trawl.

 

The stocking of the exotic species in Lake Victoria increased the total catch (Welcomme 1966, Fryer 1973), but had negative impacts on the lake ecology. The increase of stocked species populations and catches caused a decline in the haplochromine, tilapiine, and catfish landings Table 1), with which several of the exotic species compete for food resources. We know of no previous comprehensive study of the impact of exotic fish introductions on the ecosystem like that of Lake Victoria Fryer 1973). However, environmental changes caused by the introduction of exotic species have been observed in several lakes after the occurrence of irreversible changes in the food web, as reported from Laurentian Great Lakes (Smith 1968, 1972), various Scandinavian lakes (Svardson 1976), Lake Tahoe (Morgan et al. 1978), and Lake Kinneret (Gopher et al. 1983).

Changes In The Catchment Area

Changes in the Lake Victoria catchment basin (Bugenyi and Balirwa 1989, Kendall 1969) include the construction of a drainage system, vegetation removal, soil erosion, increased livestock, and recreational and industrial developments. The most severe threat is the vegetation removal by tree cutting (Harking 1987) for agricultural land development and for charcoal and firewood production. The Kenyan part of the catchment area is populated by 42% of the country's population and is drained by several rivers. Pollutants and wastes from urban centers, industries, and agricultural farmland (Ochumba 1984) are flowing into Lake Victoria via these rivers and direct runoffs (Allabaster 1981).

Natural conditions in the catchment provide high river discharges. Therefore, the impact of pollutants on the aquatic communities in the rivers and the lake itself are not effectively nullified (LBDA 1984). The degree to which these pollutants might be harmful cannot be adequately assessed because of the absence of long-term water-quality data. There are several localized parts of Lake Victoria where pollution impacts are significant and fish kills, as well as algal blooms, have been recorded (Ochumba and Kibaara 1989, Ochumba, 1990). Chabeda (1982) pointed out that loads of drained pollutants from wheat fields contain higher levels of nutrients compared to those from sugar-producing areas. River studies (LBDA 1984) indicated a general decrease of water quality from the upper catchment areas to downstream sections. The average annual nutrient input from the catchment of the Kenyan pan is 20 kg/km²/yr for total phosphorus and 400 kg/km²/yr for total nitrogen.

Present Kenyan water management legislation is administered by several governmental institutions (Moore and Christy 1978) with relatively low levels of coordination:

-The Ministry of Water Development controls water use permits and pollution prevention.

-The Ministry of Health is responsible for traditional nuisance abatement, water supply protection, and pesticide control.

-The Ministry of Agriculture is responsible for soil erosion control, provision of food, and chemical and toxic substances utilization.

-The Chief's Authority coordinates the local administrations to issue instructions for a reforestation program and the prevention of stream pollution.

-The Ministry of Environment and Natural Resources is responsible for watershed management. Efforts are under way at present that focus on pollution control, and on achieving the statutory basis for environmental impact assessments.

The influence of modifications in the catchment area on fish stocks in Lake Victoria is difficult to estimate. Nevertheless, these effects are apparently significant (Marten 1979, Benda 1979, Barel et al. 1985). Early studies show that the potamodromous fish species were more abundant in the rivers, but their densities are very reduced at present. Ancient commercial fisheries in Lake Victoria were based on migratory species that moved upstream to spawn during the rainy season when rivers were flooded (Whitehead 1958, 1959, Corbet 1961). The abundance of these fish has declined dramatically and this fishery has effectively been destroyed (Whitehead 1958, Balirwa and Begenyi 1980). The construction of dams, unrestricted dumping of industrial and agricultural wastes and other pollutants in the catchment area, reduced flows and increased sediment accumulation accompanied by deforestation, and draining of marshes and swamps has brought about the decline of this fishery. Soil erosion and increased concentrations of suspended matter also reduced algal photosynthesis and consequently fish productivity (Meadows 1980). The stocks of riverine fish populations are now at their lowest levels since the early 1960s (Rabuor 1989). Consequently, fishing potential has declined significantly, accompanied by a direct negative impact on fishermen's families, who rely on fisheries for their livelihood from Lake Victoria. There is an urgent need for effective efforts aimed at conservation of the native fish communities, accompanied by stocking operations of the endemic species (Evans et al. 1988).

Eutrophic Status Of Lake Victoria

Studies on the water quality of Lake Victoria revealed many uncertainties (Ochumba 1987). The lake is acting as a sink for most of the imported contaminants from inflowing waters that accumulate in the ecosystem, by increasing concentrations in the lake water, in the sediments, and in the biota, especially fish. About 85% of inflowing water into Lake Victoria evaporates (Talling 1966). A few comprehensive studies of water quality were carried out 30 years ago focusing on substances, but not on organic matter and heavy metal contamination. The chemical composition of lakes and rivers in Africa is predominantly controlled by atmospheric conditions (Kilham 1990). Studies of Lake Victoria's trophic status are scarce, whereas studies of fishery biology (mostly in bays) are more numerous. Recent fish kills have brought to the attention of the authorities the possible increased eutrophication of Lake Victoria.

TABLE 3 Summary of Comparative Limnological Data from Various Authors on Lake Victoria

(Range Given)

Feature

         

Author

 

Talling

Akiyama

Melack

LBDA

Ochumba/

Hecky/

 

1966

et al. 1977

1979

1984

Kibaara 1989

Mungoma 1990

Phosphate phosphorus(PO4P) ,ug/l

7.0-120

0.1-122

 

0.2-75

4.0-37.0

0.1-19

Nitrate nitrogen(NO3N) ~g/l

10-112

0.5-122

0.16-018

21-237

0.1-513

1.0-30

Sulphate sulphur(SO4S) mg/l

0.4-4.0

   

0.1-5.0

   

Silicate silica(SiO2Si) mg/l

4.0-8.0

0.2-3.0

2.0-7.9

0.1-7.6

 

0.06-072

Secchi disctransparenc y m

1.0-3.9

1.2-2.0

 

0.35-2.4

0.2-2.1

 

Primary productivity mg O2/m3

100-130

 

400-600

 

180-600

100-1400

Chlorophyll concentrations ~µg/l

0.5-22.3

2.1-8.5

 

1.8-23.5

8.0120

35.8-115.2

Algal dominance

diatoms

diatoms

diatoms

cyanophytes

cyanophytes

cyanophytes

Zooplankton dominance copepods

   

Thermocyclops

     

 

Recent comparative studies on the limnology of Lake Victoria (Table 3) emphasized the eutrophication processes (Akiyama et al. 1977, Hecky and Mungoma 1990, Ochumba and Kibaara 1989). We identified substantial increases in chlorophyll (3-10) and primary productivity (2-3) and substantial decreases in silica concentration (5-10). Sulphate concentrations are 10 times lower than the lowest concentrations measured in other large lakes in the world (0.1 mg/l). There has been a shift in the phytoplankton community towards nitrogen fixing blue-green species, fewer green algae, and the diatomid Stephanodiscus (Kitaka 1971). The zooplankton community is currently dominated by small-bodied species of copepods and cladocerans with low densities (Gopher et al., unpublished data).

TABLE 4 Dissolved Oxygen Concentrations (ppm) in "Bottom" Waters at Open Lake Locations

Date

Location

Depth

Total

DO2

Reference

   

Sampled (m)

Depth (m)

(ppm)

 

Jan. 61

deep north sta

57-60

60

1-6

Talling 1966

Feb-May 54

deep north sta

7-50

-

0

Fish 1957

Feb. 58

fake transect

45-65

45-65

0.2-2.2

Newell 1960

Feb. 61

deep north sta

57-60

60

0-4

Talling 1966

Feb. 69

N-S lake transect

55-60

60

0

Kitaka 1971

Mar 61

deep north sta

57-50

60

1-6

Talling 1966

Mar 67

N-S lake transect

55-60

55-60

0

Kitaka 1971

Mar 85

"open"

25-40

40

0

Ochumba & Kibaara 1989

Mar 89

Sta 32

30-44

44

0.8-0.3

NURP 1989

Mar 89

Sta 33

33-34

34

0.7-0.02

NURP 1989

Mar 89

Sta 100

40

40

3

NURP 1989

Mar 89

Sta 103

43-47

47

0.1

NURP 1989

Apr 61

deep north sta

55-60

60

1

Talling 1966

May 61

deep north sta

57-60

60

1-2

Talling 1966

May 86

Sta 32

30

30

5

Ochumba & Kibaara 1989

May 86

7,99,100,103

5-60

60

0

Ochumba & Kibaara 1989

Jun61

deep north sta

55-60

60

1-6

Talling 1966

Jun-Aug 84

Sta 32

30

32

1-9

Ochumba 1984

Jun-Aug 84

Sta33

40

41

0-7

Ochumba 1984

 

Table 4 represents anoxic cases measured in Lake Victoria (see also Fish 1957). In Kenya, anoxic waters have recently been found at shallower depths than before, suggesting an increase of oxygen demand in the hypolimnion. A eutrophication process, whose causes need to be determined, is now clearly evident. The decimation of the haplochromine fishes and endemic tilapias following the introduction of Nile perch may have profoundly altered the trophic status of the lake.

Conclusion

Lake Victoria exhibits unquestionable symptoms of eutrophication, including decreased water transparency, increased blue-green phytoplankton blooms, elevated nutrient concentrations, and hypolimnetic deoxygenation. Changes in the phytoplankton community altered the availability of food sources to primary consumers, although these grazers were considerably suppressed by higher trophic levels. However, the most important impact on water quality came about through fish introductions that modified the phytoplankton, zooplankton, and fish assemblages in Lake Victoria. Lake Victoria fish species are threatened by the worsening conditions in the lake itself and from the rivers in the catchment basin. Urban, agricultural, and industrial pollution, as well as the introduction of predatory Nile perch and competitive tilapiine species are suggested as the major factors for the deterioration of Lake Victoria's ecosystem. Oreochromis esculentus is almost extinct and many other haplochromid species are endangered. Current fisheries legislation and efforts aimed at reduction of soil erosion and sedimentation in the inflowing rivers in the catchment basin are insufficient for significant improvement of the present deterioration of the lake's water quality. Also, localized manual harvest of papyrus, shoreline dredging, and sand mining have not improved lake conditions.

Stocking the lake with large-bodied plant-grazing cichlids that are less vulnerable to predation by Nile perch is recommended to increase forage pressure on algae and detritus and reduce their densities. In addition, long-term monitoring of the lake and its tributary rivers will be required to enable administrative agencies to make decisions with regard to protecting the Lake Victoria fisheries resources.

Acknowledgments

We thank Messrs E. Okemwa and J. Ogari who provided facilities for fieldwork. Background information was provided by Les Kaufman and W. Cooper. Funding was provided by USAID Grant No. DPE-5544-G-SS-7075-00. The authors also thank the Office of Research, USAID, for funding the network meeting and publication of this paper.

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Percid Fish As A Tool For Restoration Of Reservoir Ecosystems, Improvement Of Water Quality, And Optimization Of Fishery Yield

M. Zalewski

Department of Applied Ecology, University of Lodz

Lodz, Poland

Abstract

In temperate lowland reservoirs, percid fish are one of the most important components of the ecosystem. They control the density of the large filter-feeders, such as Daphnia, which in turn eliminate algae and thus improve water quality.

Over a six-year period, the density of young of the year (YOY) perch was studied in a lowland reservoir. Yearly densities were established in correlation with late spring water levels when littoral zone resources were available. In years of high YOY perch densities, there were sharp reductions in large zooplankton and increases in phytoplankton. These conditions were detrimental to water quality and reduced the growth of YOY perch and pike-perch, which in turn reduced their winter survival rates. Further, the elimination of YOY pike-perch reduced their predatory control of pelagic plankton-feeding cyprynids and negatively influenced the zooplankton:phytoplankton ratios in the reservoir.

This study demonstrated that water quality and fish yields in eutrophicated lowland reservoirs can be restored, to a large extent, by the control and enhancement of percid fish.

Introduction

From a historical perspective, three overlapping stages can be distinguished in the development of ecology as a science. The first was focused on a description of ecosystems. The second began when a certain amount of knowledge was acquired about the structure of ecological communities and different levels of their organization. This stage focused on analysis of ecosystem dynamics in terms of the density of plants and animals and their biomass and energy content. Progress in the first two stages gave ecologists a certain predictive ability, which allowed them to start attempts at the third stage, i.e., the manipulation of the biotic structure of ecosystems for the purpose of directing the ecological process in ways that enable optimal utilization of the resources of that ecosystem by humans.

An example of such a sequence of events is a biomanipulation theory, pioneered by Hrabacek et al. 1961. This concept is based on the assumption that large filter-feeders (Daphnia) in most situations are able to maintain the biomass of phytoplankton at a low level, thus maintaining water quality. However, the drawback of this process is that Cladocerans are a most attractive food for many fish species.

The potential improvement of the water quality by biomanipulation is especially important in the case of reservoirs, which are the main sources of drinking water for large human communities. In the case of reservoirs, two groups of fish are supposedly the most efficient in eliminating filter-feeders and nektonic zooplanktivores, such as Alburnus alburnus, and juvenile perch and cyprinids. In most temperate reservoirs, the juvenile fish play the prevailing role in this process because there usually exists a strong population of efficient pelagic predatory pike-perch, which efficiently control the density of planktivorous pelagic species. The juvenile fish are less vulnerable to predation at some stages because their preferred habitat is in the shallow littoral zones with dense vegetation (e.g., flooded terrestrial vegetation not easily accessible for pike-perch). Biomanipulation can improve water quality by the "top down effect" (Northcote 1988). This means that through the introduction of predators, planktivorous fish pressure on large filterfeeders (Cladocerans) is reduced. Until now this has been done mostly in lakes; however, experimental evidence exists that it can be successful in some types of reservoirs. Fish fry may be the most important factor in reducing Daphnia density (Holcik 1977, Zalewski et al. 1990a) and the density of fish fry (reproductive success) is dependent on water level (Martin et al. 1981, Zalewski et al. 1990b), i.e., through shoreline habitat and food resource utilization (Ploskey 1986).

From the trophodynamic point of view, efficiency of biomanipulation increases when the number of trophic levels between the level manipulated and phytoplankton decline (McQueen 1990). Also, biomanipulation has a greater chance of success at low and intermediate levels of eutrophication, and with the physically unstable epilimnion which drives algal communities into the early successional stages (Reynolds 1987). At this stage, algae are usually characterized by small forms such as Chlorococcales and Bacillariophyceae (Puchalski 1990), which are easily filtered by Cladocerans.

All three of the conditions cited above can be fulfilled to a greater extent by reservoirs than by lakes. This is because in reservoirs the density of zooplanktivorous fish fry can be controlled by water level manipulation. Eutrophication can, to a certain extent, be slowed down by release of nutrient-rich hypolimnic waters. In addition, the physical instability of the epilimnion may be enhanced not only by wind action, which can be amplified by reduction of shoreline ecotone complexity, but also by pulsing water release.

PERCID FISH AS POTENTIAL TOOL FOR

RESTORATION OF RESERVOIR ECOSYSTEMS

Reservoirs are traps for nutrients carried down by the river; thus, they possess an inherent tendency to eutrophicate. The biomanipulation technique of restoring water quality by manipulating biotic components increases the density of large efficient filtrators such as Daphnia, thereby reducing manyfold the concentration of phytoplankton. To achieve this effect, the planktivorous fish (cyprinids and perch) densities must be reduced by predators, such as pike-perch or walleye. This process, in terms of energy flow through the trophic structure of an ecosystem, can be described as reconstruction (restoration) (Figure 1).


FIGURE 1. Use of percid fish for reservoir ecosystem restoration by manipulating trophic pathways for the improvement of water quality and fish yelds. Unrestored (a) and restored (b) reservoir with pike-perch enhanced and perch reproduction controlled (see explanation in text).

In oligotrophic systems (good water quality, low production) the largest portion of energy is diverted from primary producers (algae) to subsequent trophic levels (planktivorous and predatory fish). In eutrophic systems, characterized by high primary productivity, most of the phytoplankton production is not consumed by filtrators (Figure 1). This is because large Daphnia are eliminated by dense populations of juvenile percid and cyprynid fishes (Zalewski et al. 1990b).

The dead algae nutrients are recirculated by the "microbial loop" and a continuing high nutrient supply maintains concentrations of algae, resulting in deterioration of water quality. Benthic detritivores may be stimulated in the system as long as the bottom oxygen supply of the reservoir is sufficient. In such a system (Figure 1) the large part of the energy is accumulated in bottom sediments leading to a decrease of the aquatic habitat quality. Figure 1 demonstrates the restoration of a reservoir by enhancing the predatory pike-perch, which reduce pelagic, zooplanktivorous fish populations (in this case perch, Perca fluviatilis). In reservoirs, pike-perch or walleye is the most important biomanipulation tool because the second important predator of temperate zone lakes, the northern pike, Esox lucius, needs a diversified littoral zone as an optimal habitat. This rarely exists in reservoirs due to water level fluctuations.

On the basis of initial research (Zalewski et al. 1990a,b) and other published data (Gulati et al. 1990), it can be hypothesized that, in reservoirs, the most efficient way to restore water quality and fish yield would be to enhance pike-perch populations to control pelagic zooplanktivorous fish. From a dynamic point of view, such restoration can be described in terms of changes of the energy flow pattern from detrital food chain characteristics of eutrophic ecosystems (Figure 1A) to mesotrophic, high water quality and high fish yield ecosystems (Figure 1B). Using pike-perch or walleye as a biomanipulating tool, it may be possible to restore conditions of the ecosystem. If true, the advanced stage of eutrophication in many reservoirs can be reversed, or at least seriously reduced. Restoring aquatic habitat quality should provide time to create strategies for the conservation of the catchment area. These strategies should include sewage treatment plants and the reduction of aerial pollution.

Perch Recruitment, Water Quality, And Predator Dynamics

Some of the assumptions discussed above were confirmed in practice at a lowland reservoir in central Poland. The Sulejow Reservoir is situated in the middle course of the Pilica River, a tributary of the Vistula River. Its area is approximately 20 km² and the average depth is 3.3 m.

Galicka and Penczak (1989) evaluated annual nutrient loadings at the beginnings of the eighties at a level of 30 g N (total) and about 5 g total P per m³. Detrital sedimentation occurs mostly in the upper eutrophic part of the reservoir and is separated from the main part by a kind of bottleneck. The oxygen level is usually high due to water mixing by wind and algal photosynthesis. In 1984, a stable high water level during spring and summer was responsible for the demographic explosion of perch and cyprinids, which are major components of the fish community. Analysis of data on perch fry densities in the littoral zone and in changing water levels during the spawning and post-spawning periods (May, June, July) demonstrated a correlation between an "area stability index" (As) (Zalewski et al. 1990b) and perch fry density (Figure 2).


FIGURE 2. Relationship between perch reproductive success (fry density in the littoral zone in mid-july) and water level (area stability index).

During the year of the highest and most stable water level (1984), a high density (mean 37 per m²) of perch fry was observed in the shoreline zone. At that time the zooplankton density was drastically reduced compared to former years (1982, 1983) when perch density did not exceed 5 per m² (Zalewski et al. 1990 a,b). This reduction of zooplankton, mostly large Cladocerans, was reflected in the stomach contents of the perch and in their foraging strategy. During the year of low perch density, when Cladocerans were abundant, a distinct feeding peak (stomach contents [weight] measured as a percentage [4%] of perch fry body weight) occurred in the evening when Cladocerans migrate to the surface and to the littoral zone (Figure 3). But during the year of very high perch fry density, they fed continuously, almost 24 hours a day, and drastically reduced the density of Cladocerans (Figure 4b). At that time, the perch fry were eating fewer Copepods and perch stomachs were only approximately 1% full (Figure 3).


FIGURE 3. Decline in perch fry growth (Instantaneous Coefficient of Growth) in the years of high YOY densities, resulting from the reduction of optimal food resources (Cladocerans), as measured by low stomach content and small amount of Daphnia eaten during 24 hours. Wf/Wb means the ratio between weight of food and fish body weight.

The drastic reduction of Cladoceran biomass was reflected in water quality. During the year of high density of filter feeders (1983), the amount of suspended matter doubled, despite the fact that Cladoceran biomass decreased almost eight times. Consequently, the ratio of zooplankton changed from 2:1 (1983) to 1 :15, because of an intensive algal bloom. Obviously the sharp reduction in the availability of optimal food was reflected in the perch growth rate (Figure 3). In years when average density was below 8 perch specimens per square meter of littoral zone (e.g., 1983), the Cladocera population was not severely reduced (Figure 4a) and served as a main food component, and helped maintain a high juvenile perch growth rate. When the perch fry density exceeds approximately 10 specimens per m², a sharp decline in the instantaneous coefficient of growth was observed. This phenomenon was a result of a break in the dynamic equilibrium of Cladoceran population density and a shift in perch fry diet to non-optimal prey, Copepods and benthos. These bad feeding conditions were confirmed by broad size ranges of juvenile perch and cannibalism observed among the O+ age group.


FIGURE 4a. Annual changes in zooplankton biomass in the year of low perch fry density (1983)


FIGURE 4b. Annual changes in zooplankton biomass in the year of very high perch fry density (1984).

The sharp reduction in large zooplankton density was reflected in a greater pike-perch fry growth than that in perch. In mid-July 1982, they achieved 5.5-8 cm, but in 1984 only 3.3-4.2 cm. In both years the temperature ranges and patterns during late spring and early summer were similar. The fry sizes always decreased upstream in the more eutrophic part of the reservoir. The very poor growth of juveniles in 1984 was due to lack of optimal prey, Cladocerans. As a result, pike-perch were late in achieving the size at which they become piscivorous (3.8 cm). The potential prey-fry of perch and cyprinids were too large at that time; consequently, 43% of the pike-perch had empty stomachs, and all were in poor condition. It is a well-known phenomenon that poor fry growth during summer can lead to low winter survival. Loss of one generation of this easily overexploited, short-lived species seriously reduced its populations for many years. The elimination of predator control in the fish community of a reservoir, due to intraspecific competition at the early fry stages, may have great effect on the dynamics of pelagic planktivorous fish, which in normal years are at very low densities because of efficient pike-perch predation. Pelagic zooplanktivores may react to a sharp decline in predator numbers by a rapid increase in density and increased feeding pressure on zooplankton. In addition, such-fast growing populations of pelagic fish, in contrast to perch fry communities, will be impossible to control by water level changes. These results are summarized in Figure 5, and clearly show that in a lowland reservoir it is possible to achieve concordant maintenance of water quality and fishery yield by control of water level. However, it should be expected that this regularity will be disturbed when the structure of upper trophic levels is sharply modified (e.g., by the drastic reduction of the main predator) and a shift occurs in the main mechanism regulating zooplanktivorous fish pressure on filter feeders in the shoreline ecotone and open water areas.


FIGURE 5. Effect of spring water level on lowland reservoir water quality and fish yield.

Discussion

Looking at this process in a broader context, one can say that the general mechanisms described can be expressed in terms of general ecological theory. Fish tend to be "r" selected organisms, which means they have an ability to provide large numbers of offspring when environmental conditions are favorable.

Usually, in ecosystems where abiotic control mechanisms prevail (Zalewski and Naiman 1985) biotic structure is simple and feedback control mechanisms, such as predation and intraspecific competition, are not well balanced.

In the case described above, the perch population took advantage of the high water level, and with its opportunistic reproductive style (Baron 1975) it dominated energy flow and nutrient distribution, which put the shoreline ecotone in an early succession stage of an aquatic ecosystem. These data demonstrate that the explosion of one species may occur when even part of an ecosystem is modified. Such a warm, highly diversified, trophically rich ecotone, without specialized predators, is created periodically in reservoirs as a result of water level changes. This is especially true in lowland reservoirs where a relatively small water level increase creates large inundated areas rich in food that are warm and safe from limnetic predators. The above data confirm the hypothesis of Naiman et al. (1989) about the importance of land-water ecotones in landscape process dynamics.

This process also supports the view of Patten and Odum (1981) concerning the cybernetic nature of ecosystems, that is, an advantage achieved by one species in a relatively mature system (10-year-old reservoir) may occur only when abiotic factors are changing the dimensions of the resources. Thus, this would be an ecotone without predatory pressure, and with a diversified and rich food supply (Ploskey 1986). An analogous reaction of species to the extension of resources has also been described for another species, Perca flavescens (Thorpe 1973).

Despite the great reproductive success of perch in 1984, which was a rather catastrophic event from the point of view of the homeostasis of an ecosystem, the ecosystem still possesses a tendency toward equilibrium. This was expressed by a sharp reduction in growth that usually results in a mass winter mortality of any cohort that exceeds the carrying capacity of the given environment. In a stable and trophically limited ecosystem, such as Scottish lakes, the perch population demonstrates a feedback control of its own population density by cannibalism. The juvenile perch form 14-30% of the adult fish diet (Thorpe 1977).

The sharp reduction of basic food resources by one species, described above, that resulted in densities exceeding the carrying capacity of its environment, also impacted on the predators' recruitment and long-term population density. Under natural conditions, pike-perch occur as a multigenerational population and thus, presumably, do not react sharply to a population reduction in one generation. However, in the Sulejow Reservoir, intensive angling pressure seriously reduces the life span of the predator population. Thus, human impact amplifies the ecological effect of the resource's reduction by competing species.

Juvenile pike-perch are especially vulnerable to a decline in food resources, because juvenile

percids consume up to 32% of their body weight in food a day. Thus, a reduction in the density of

crustaceans below 100 per lifer stimulates high percid mortality (Li and Mathias 1982). That is why

stocking of good quality pike-perch, above 4 cm in length, that can feed on cyprinid fry might be of

great importance for enhancement of this species and controlling the density of plankton-eating fishes.

Finally, does the relation described above, involving water level, fry density, and water quality, occur in all types of reservoirs and make the manipulation of the biotic structure of reservoir ecosystems a future management tool for water quality and optimization of fishery yield? Obviously, the answer depends on the type of reservoir. The shape of the reservoir valley, complexity of ecotonal vegetation cover, and eutrophication stage will be the major factors. These will determine the shift in resource dimensions during water level changes that affect fish community dynamics, as well as the "top down" effect resulting from water quality.

Acknowledgments

The author thanks the Office of Research, USAID, for funding the network meeting and the

publication of this paper.

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